3
Analytical methods
Coagulation and flocculation of wastewaters
Chemical coagulants are added to wastewaters for the primary purpose of removing suspended solids and also phosphorus.
The addition of chemicals reduces also heavy metal concentrations and improves disinfection efficiency. Chemical
coagulation can be obtained by adding to wastewaters defined amounts of lime (calcium hydroxide), alum (aluminium
sulphate) or iron salts (ferric or ferrous). The coagulation-sedimentation process typically involves:
a) injection and mixing of the coagulant that neutralize the electric charge, prevalently negative, on suspended particles.
Good and rapid mixing of the coagulant and wastewater is important to ensure efficient use of the chemical. Typical
detention times in plant basins for turbulent mixing are 15 to 120 seconds.
b) agglomeration of the coagulated particles into large settable flocs. This agglomeration is accomplished by stirring the
water slowly so to allow the solids formed by the addition of chemicals to grow in size so that they can be removed by
gravity settling. The stirring is usually performed by slowly rotating paddles during 10 to 30 minutes.
c) sedimentation of the flocculated material by gravity separation. The solids are allowed to settle by gravity from the
chemically treated standing wastewater. The effluent from the chemical settling tank is very clear and does not usually
require further filtration.
The quantity of chemical coagulant required to achieve the test varies with time and from wastewater to wastewater.
Typical coagulant doses are: 75 to 250 g/m
3
for alum
, 45 to 90 g/m
3
for ferric chloride, 200 to 400 g/m
3
for lime.
Reagents
The choice and dosing of the chemical coagulant to be adopted for the removal of suspended solids from wastewaters
derive from the results of laboratory evaluations by the so called Jar test. Multiple stirrers with reproducible stirring speeds
allow to adopt standard conditions for the test, that are the basic requirement for reproducible results.
The standard conditions are related to:
- dimensions and shape of vessel and stirring paddle
- volume of wastewater
- time and speed of stirring during turbulent mixing
- time and speed of stirring during flocculation
- evaluation criteria for the obtained results.
The most diffused adopted conditions are:
- 1000 ml glass beakers (jars), tall form, Ø 105 mm
- 600 ml wastewater samples
- stirring paddles 25 mm high, 75 mm wide, 1 mm thickness
- stirring height of paddles, middle height of sample
- turbulent stirring after chemical addition: 120 rpm, 120 seconds
- slow speed flocculation: 30 rpm, 25 minutes
- first evaluation of results after 5 minutes of sedimentation
The results can be evaluated on the basis of different criteria:
a) floc dimension evaluation with numerical degree (0=no flocs, 2=very small, 4=small, 6=medium, 8=large, 10=very large).
b) time from the addition of chemicals to first appearing of flocs.
c) evaluation of residual turbidity of supernatant, after a determined sedimentation time, by turbidimeter.
d) measurement of electro-kinetic potential of suspended particles on a sample taken immediately after the addition and
mixing of chemicals. Coagulations should be easier when zeta potential of particles is very low.
e) evaluation of filterability of clarified water by standardized membrane filters under pressure. The reduction of water flow
with time is related to the degree of clogging of filters due to residual unsettled suspended matter.
Other measurements are related to the pH of water after chemical treatment (lime raises the value while iron and aluminium
salts lower it) and the temperature of water at which the experiments are performed.
References:
American Society for Testing Materials Norm ASTM D-2035-G4 T; Passino R. and M.Beccari (1970)
“Standardization of jar test in coagulation-flocculation processes for the removal of inorganic turbidities (in Italian). Acqua e
Aria, Milano, 1-10, October 1970
”; Degremont (1978) “Etude de la coagulation et de la floculation des eaux. Methode n.703,
948-950. Memento technique de l'eau. 1200 pages, Paris
”; U.S. Association of Environmental Engineering Professors
(1972)
“Environmental Engineering Unit Operations and Unit Processes Laboratory Manual. J.T. O'Connor Ed. 350 ppg”.
Leaching test
Leaching tests are used to simulate the behaviour of a waste in a dump that is submitted to the leaching activity of rain
water. There are two different methods that use leaching solutions of different type:
a) acetic acid, proposed for dumps receiving both organic and inorganic wastes;
b) carbon dioxide saturated water, for dumps receiving only inorganic wastes.
Waste sample to be examined
: if a liquid phase is present it must be removed by filtration or centrifugation and preserved
in a refrigerator at 2-4 °C. The granulometry of the solid phase, must be lower than 9.5 mm (standard sieve).
Waste extraction
: 20-25 g of the solid phase are weighed with a precision of ± 0.1 g.
Method a
) A volume of distilled water corresponding to 16 times the weight of the sample is added.
The pH of the mixture is kept to 5±0.2 using 0.5 N acetic acid. The stirring is performed for 24 hours, controlling the pH
every 15 minutes during the first hour and then every hour. The pH is corrected to 5±2 using again 0.5 N acetic acid. If after
24 hours of stirring the pH is higher than 5.2, a new correction is made and the stirring prolonged for other 4 hours.
Method b
) The leaching solution is given by a volume corresponding to 20 times the sample weight of distilled water
saturated by carbon dioxide for not less than 15 minutes. The stirring is performed during 6 hours without any pH correction.
Stirring speed
: must be able to maintain continuously mixed the solid phase with the leaching solution.
Analysis of extract
: the leaching solution and the liquid phase if present in the original sample are filtered by a membrane
filter of 0.45 µm porosity and submitted to analysis for the single metals by the methods commonly used for waters.
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